As communications systems have developed and both the desired and actually used information transmission rates have increased, more attention has been paid to the development of optical communications systems. As presently contemplated, such systems will use a light source and a photodetector that are optically coupled to each other by a glass transmission line which is commonly referred to as an optical fiber. In fact, the communications art has developed to the point where such systems are now used commercially.
Two types of devices, lasers and light emitting diodes, are presently used as light sources for optical communications systems. Lasers are more attractive candidates than are light emitting diodes for light sources at high information transmission rates because they emit radiation over a narrower wavelength region and problems associated with, for example, material dispersion are reduced. Lasers used as light sources in fiber based optical communications systems should, of course, be reliable, as measured by, for example, device lifetime, and possess device characteristics, such as threshold currents, I.sub.th, and external differential quantum efficiencies, .eta..sub.D, that are relatively insensitive to temperature variations.
As might be expected, system efficiency depends upon many factors including the efficiency of optical coupling between the light source and the fiber. Although lenses may be used to increase the optical coupling efficiency between the laser and the optical fiber, it is still desirable that the laser beam divergence be narrow, especially in the direction perpendicular to the junction plane of the laser, to maximize coupling efficiency between the fiber and the laser. A widely divergent beam usually results in inefficient coupling of optical power into the optical fiber. Less optical power in the optical fiber results in less economical system operation as repeater spacings must be reduced, photodetector efficiency increased, etc. Thus, lasers with relatively narrow beam divergence are desirable. Relatively narrow as used in this specification means a half power full width of less than 50 degrees.
Although many laser structures have been considered as candidates for optical communications systems, the structure known as the double heterostructure laser now appears to be the leading candidate as the light source for such systems. At wavelengths of approximately 0.8 .mu.m, these lasers are generally made with (AlGa)As material system. At longer wavelengths, for example, 1.3 82 m, these lasers are generally made with InGaAsP. Good temperature stability of the threshold current and the external differential quantum efficiency may be obtained by having a large step in the AlAs composition between the active and cladding layers. For example, if the composition of the layers is represented by the formula Al.sub.x Ga.sub.1-x As, x changes by at least approximately 0.3, that is, .DELTA.x=0.03, between the active and cladding layers. A large .DELTA.x increases the barrier height and reduces the carrier leakage into the cladding layers. This makes the threshold current of the laser less sensitive to temperature variations which is especially important for operation at high temperatures.
However, as the compositional, and thus the refractive index, step increases, the beam divergence also increases. While this increased divergence could be reduced by using a small .DELTA.x between the active and cladding layers or by having very thin active layers, both of these approaches have at least one drawback. A very thin active layer may lead to an undesirable increase in the threshold current and a small refractive index step reduces the carrier confinement in the active layer and leads to a temperature sensitive threshold current and poor device lifetime. Double heterostructure lasers commonly used in light optical transmission systems with an active layer between 0.15 and 0.2 micrometers in thickness and a .DELTA.x of approximately 0.3 have a half power full width of approximately 50 degrees.
Double heterostructure light emitting devices have been proposed in which compositional variations of at least one layer results in focusing of the light output from the laser. For example, U.S. Pat. No. 4,152,044 issued on May 1, 1979 to Yet-Zen Liu describes a double heterostructure AlGaAs device in which compositional grading of a core layer that is adjacent to the active layer results in a graded index of refraction that causes the light to focus periodically. The core layer has an Al concentration that decreases continuously from the coreclad interface to the core-active layer interface. Thus, cladding layers have an Al concentration at least equal to that of the core. This device, however, is not completely satisfactory because the structure will result in less efficient current confinement. Additionally, the described device requires relatively thick, i.e., thicker than a wavelength of the emitted radiation, graded layer so that the radiation is focused.